The oil wells are formed by oil or gas deposits, solid residues and water, enclosed in rocky or sandy formations. These well bores may have different depth levels, varying from superficial to shallow, middle or deep. Once the well has been drilled and depending upon the permeability of the medium, the extraction of the oil or the gas that comes out of the perforation is initiated through natural or artificial fractures in the rock until it reaches the surface, generally through metallic tubing. As a function of the permeability of the medium, the flow of oil or gas may be very small requiring artificial fracturing measures in order to increase it. Therefore, once the drilling phase has been completed it is possible to initiate the extraction of gas or oil, or else to employ fracturing techniques using proppants to initiate the extraction.
Also, as time goes by the continuous passage of oil or gas through the natural or artificial fractures begins to drag solid residues which gradually fills them, finally clogging or reducing the spaces within the rock, thus decreasing the oil or gas flow with the consequent reduction of productivity of the well. This leads to such critical situations that it becomes necessary to interrupt the operation of the oil well due to the lack of operation economy.
Several techniques have been developed with the purpose of improving the productivity of recently drilled wells, or of rehabilitating wells that already have clogged passages, or even of improving the productivity of wells still under operation. Those techniques which are called hydraulic fracturing consist of injecting fluids enriched with high-resistance solid agents into the existing oil wells or wells being drilled. This causes the formation of fresh fractures in the rocks that are filled up with said solid agents, thus preventing the fractures from being clogged by the external pressures exerted at the time when the pressure used in the fracturing process is ceased. Once the new fractures are opened and filled up, the oil or gas flows more easily, passing through the fractures filled with solid agents.
Such solid agents called proppants must have sufficient mechanical strength to resist to the pressures exerted on the fracture without breaking, must be resistant to the high temperatures found in the medium where they will be used, must be as spherical in shape as possible and they must also have very adjusted granulometric sizes in order to guarantee as much permeability and conductivity of the filled-up fracture medium as possible. In addition, since their use environment comprises several corrosive agents, the proppant must also be resistant to corrosion.
Various solid materials are already conventionally used as proppants, such as: sands, resinated sands, steel shot, glass spheres, in addition to different kinds of synthetic ceramic materials. Each of those proppants has advantages and disadvantages and has been used in numberless wells throughout the world.
A number of patent documents refer to these materials and manufacture processes and use of ceramic proppants, among which the following are pointed out: BR 8903886, BR 9501449-7, BR 9501450-0, BR 9502864, BR 9700995, U.S. Pat. No. 3,491,492, U.S. Pat. No. 4,068,718, U.S. Pat. No. 4,440,866, U.S. Pat. No. 4,427,068, U.S. Pat. No. 4,443,347, U.S. Pat. No. 4,522,731, U.S. Pat. No. 4,522,735, U.S. Pat. No. 4,555,493, U.S. Pat. No. 5,464,459, U.S. Pat. No. 4,623,630, U.S. Pat. No. 4,639,427, U.S. Pat. No. 4,644,819, U.S. Pat. No. 4,658,899, U.S. Pat. No. 4,688,645, U.S. Pat. No. 4,713,203, U.S. Pat. No. 4,717,594, U.S. Pat. No. 4,746,468, U.S. Pat. No. 4,879,181, U.S. Pat. No. 4,894,285, U.S. Pat. No. 4,921,820, U.S. Pat. No. 4,921,821, U.S. Pat. No. 4,944,905, U.S. Pat. No. 4,977,116, U.S. Pat. No. 5,171,133, U.S. Pat. No. 5,188,175, U.S. Pat. No. 5,120,455, U.S. Pat. No. 5,325,921, JP 5784731, EP 0083974, EP 0112350, EP 0116369, WO94/09454, DE 2921226, DE 29218584, DE 3617340, GB 2037727, FR 2090561, FR 2437689, and others.
However, none of the above documents relates to ceramic proppants developed for the purpose of reducing or even eliminating the “flow-back” process.
For the purpose of this patent application it is important to define that the term “flow-back” should be understood as the loss of the proppant applied in the fracturing immediately after cleaning the well to begin its operation, or even as the proppant that leaves the well together with the gas or oil throughout the production process. This phenomenon is known since long ago but it has only recently drawn the attention of the fracturing process users. This loss of proppant material placed in the fracture may cause environmental problems and even significant cost problems involved with the loss of all the proppant placed in the fracture. The “flow-back” effect is a long-term process, with volumes that cause serious operational problems and that is difficult to predict.
The main factors that cause loss of proppant due to the “flow-back” may be connected to:
i) low closure pressure of the fracture;
ii) work environment with a high pH maintained for a long time;
iii) action of excessive forces acting in parallel to the axis of the proppant package;
iv) incorrect choice of the proppant;
v) false economy, that it to say, the use of proppants that have more economical unit cost but are not suitable for the desired operation;
vi) fracturing design below the desired optimum.
Ceramic proppants are particularly useful for low-pressure wells due to the high permeability that is achieved with this kind of proppants. The fact they present almost perfect sphericity is a very important factor in obtaining high-permeability fractures. On the other hand, such ceramic proppants favor situations of loss of material due to “flow-back” exactly because they present this high sphericity. Low-pressure wells (shallow wells) associated to highly spherical proppants and a very rapid cleaning of the well before an efficient stabilization is achieved lead to the trend of a destabilization of the column with the transportation of the proppant to the surface.
Prevention of production by “flow-back” requires a proppant packing capable of resisting to the modifications of forces induced during normal productions. Simultaneously, the ceramic package should be capable of resisting to compression and rearrangement of the forces induced by intermittent cyclic productions.
The solutions known from the prior art for decreasing, preventing or even eliminating the “flow-back” effect comprise the use of resinated proppants, wherein the resins maintain the grains together, thus imparting difficulty for them to flow out of the fractures.
As examples of patent documents wherein resinated proppants are already disclosed for use in decreasing the flow-back effect, the following may be cited: U.S. Pat. No. 6,528,157, U.S. Pat. No. 6,311,773, U.S. Pat. No. 6,116,342, U.S. Pat. No. 6,079,492, U.S. Pat. No. 5,924,488, U.S. Pat. No. 5,908,073, among others.
It is important to address to the difference of the “flow-back” when non-resinated proppants are used, wherein the undesirable effect almost always occurs due to the structural instability of the proppant itself, from the “flow-back” of resinated proppants, wherein it occurs due to design problems of the fracture or for any other reason.
Lately, the use of resinated proppants for any type of well and fracture has been indiscriminately recommended. However, although the resins contribute for eliminating or decreasing the “flow-back”, they decrease the fracture permeability, consequently decreasing the productivity of the well, in addition coupled to their low resistance to high temperatures and lower crushing strength by the action of closure pressure.
Serious problems have also been observed in using these types of resinated proppants related to the total loss of the proppant that remains on the walls of the fracture due to a separation the resin from the substrate.
Other documents from the prior art relate to ceramic proppants associated to metals such as document GB 2,359,316 which teaches a composition comprising a mixture of proppant and a deformable material, for example, aluminum.
The objective of the present invention is to provide a proppant composition that may be used in an effective way for decreasing or even eliminating the “flow-back” effect without the drawbacks of the proppants already known from the prior art for this purpose.